This invention relates to an emission control apparatus for achieving quick light-off of exhaust gases found in applications involving automotive catalytic converters, diesel particulate filters, industrial stacks, wood stoves and other applications in which the initial temperature of the exhaust gas stream is too low to initiate fast light-off of the catalyst. In particular, the invention relates to an apparatus for reducing the time, and the amount of electrical energy required to attain catalytic light-off of a cellular honeycomb structure when such structure is used as a catalytic converter for internal combustion engines.
The Federal Test Procedure (FTP) is an emission certification test used on light-duty vehicles. Cold-start under FTP standards is the starting of an engine after a period of 12 to 36 hours of non-operation in a temperature environment of 20.degree.-30.degree. C. For most cold-starts, there is a time span of one to two minutes between the time the emissions begin, that is, cold start, to the time the substrate heats up sufficiently for catalyst "light-off" to occur. It is estimated that about 50% of the pollutants escaping into the atmosphere from a vehicle equipped with a catalytic converter is generated in these first two minutes following cold start-up. Light-off time is the time it takes to achieve a 50 percent conversion efficiency. The temperature at which about 50% of the pollutants have been converted to harmless gases is the light-off temperature. For most emission control systems, the light-off temperature is generally in the range of about 150.degree. to 220.degree. C. for fresh (unaged catalyst), to about 230.degree. to 380.degree. C. for aged catalysts. By reducing the light-off time, the amount of pollutants escaping into the atmosphere can be reduced.
Pollutants from automotive exhaust systems continue to present a substantial environmental challenge. Catalytic converters have been successful in reducing automotive exhaust pollution. However, as pollution standards become more stringent, there continues to be an increasing need for better and improved converters, particularly during cold-start. When a catalytic structure is heated to a sufficiently high temperature, the pollutants are catalytically reacted to produce harmless gases.
Numerous schemes have been proposed to provide faster light-off and thereby reduce the amount of pollutants discharged into the atmosphere during cold-start. Mainly, it has been suggested to heat the entire catalytic converter or to heat the inlet gas stream in order to bring the substrate to its light-off temperature faster. Such heating methods are inefficient and require considerable amounts of energy. According to one of these modifications, a metal heater is disposed in the exhaust gas passage ahead of the catalytic converter. The heater first heats up the exhaust gas which then heats up the catalytic converter. The problem with this method is that a substantial amount of electric power is required to raise the metal substrate to a temperature high enough to heat all of the incoming exhaust gases. Recently, it has been suggested to preheat a cellular structure for the purpose of achieving early light-off by forming a resistance heater directly onto the surface of a catalytic structure and providing means for activating the surface by passage of electrical current or other heating means for the purpose of heating the structure.
An engine exhaust system has also been suggested in which a by-pass duct is connected in parallel with the catalytic converter to form a closed recirculation loop around the converter. Within the by-pass duct there are arranged, a fan and a heater which operate to draw air downstream of the converter into the duct to be heated and returned to a region upstream of the converter.
More recently, it has been suggested to introduce a metered volume of comminuted water-soluble, oxygen-containing organic compound such as alcohol, aldehyde, ketone and ether, into a NO.sub.x -containing gas in the presence of excess oxygen prior to exposure to a copper-containing ZSM5 zeolite.
It has also been suggested to inject a mixture of fuel and air into the exhaust stream so that the exhaust stream runs rich, and to then ignite the mixture, using a spark plug, before the gas reaches the catalytic reactor. In this system, the air is supplied by an electric pump. While this system reduces the high energy (battery drain) problems associated with the metal heaters discussed above, it requires a high concentration of combustible gas, injector, and air pump for proper operation. More significantly, there is a risk of overheating the catalytic reactor which could cause the catalyst to degrade or cause the substrate to melt or deform.
There continues to be the need to provide more efficient cold-start exhaust systems. Accordingly, it is the object of the present invention to reduce the electrical energy needed to produce quick light-off of catalytic reactors.